1. Field of the Invention
The present invention relates to an X-ray mask, in which an X-ray absorber is selectively formed on an X-ray transmissive layer, and a fabrication process therefor.
2. Description of the Related Art
Conventionally, an X-ray mask has a configuration, in which an X-ray absorber is formed into a predetermined pattern on an X-ray transmissive layer. FIG. 1 shows the conventional X-ray mask. On the upper surface and lower surface of a support frame 12 made of Si, an X-ray transmissive layers 13 and 14 made of SiC or SiN are formed. On the X-ray transmissive layer 13 on the upper surface of the support frame 12, an X-ray absorber 15a which is made of W or Ta is formed in a device pattern. As the X-ray absorber, WTiN is disclosed in Jpn. J. Appl. Phys. 31 (1992) pp. 4210-4214 (H. Yabe, et al.). Also, Ta.sub.4 B is disclosed in J. Vac. Sci. Technol. B7(6), 1989, pp 1561-1564 (M. Sugawara, et al.). An alloy of Ta, Al, Ti, Si and Mo is disclosed in Japanese Unexamined Patent Publication (Kokai) No. Heisei 2-2109.
One example of the fabrication process of such X-ray mask is illustrated in FIGS. 2A to 2D. At first, as shown in FIG. 2A, on upper and lower surfaces of a Si substrate 12a, SiC layers 13 and 14 are formed. Also, as shown in FIG. 2B, an X-ray absorbing layer 15 is deposited by way of sputtering method on the upper surface of Si substrate 12a. Also, over the X-ray absorbing layer 15, a photoresist mask 16 is formed by applying an photoresist on the X-ray absorbing layer 15 and subjecting the photoresist with exposure in a desired pattern and development.
Then, as shown in FIG. 2C, using the photoresist mask 16, the X-ray absorbing layer 15 is etched into a predetermined pattern to form an X-ray absorber 15a.
Thereafter, as shown in FIG. 2D, the lower surface of the Si substrate 12a is subject to an anisotropic etching to form the support frame 12.
It should be noted that, in such X-ray mask, it has been required to have long period stability in the X-ray absorber. Therefore, as shown in FIG. 2D, the surface of the X-ray absorber 15a is oxidized to cover with an oxide layer 17 (for example, Japanese Unexamined Patent Publication No. Heisei 3-173116, Japanese Unexamined Patent Publication No. Heisei 3-116716, Japanese Unexamined Patent Publication No. Showa 61-292919).
In such prior art X-ray absorber is made of a metal. Therefore, for certainly stabilizing the X-ray absorber, oxidation of the surface is performed. However, a volume of the X-ray absorber causes expansion to vary the internal stress of the X-ray absorber by the oxidation. For example, in case of Ta, oxidation propagates in the depth up to 10 to 20 nm for a several tens minutes after deposition. When the thickness of the X-ray absorber is 0.5 .mu.m, internal stress of the X-ray absorber is compression stress of 10 to 20 MPa (J. Vac. Sci. Technol. B11(2), (1993) pp 301-303 (Y. Yoshihara and K. Suzuki). Therefore, due to the internal stress, the pattern dimension and position of the X-ray absorber should be changed to cause error and reduce patterning accuracy.
For such problem, even if a control for reducing the stress is done during process for forming the layer of the X-ray absorber, upon patterning of the X-ray absorber, new surface of the X-ray absorber should be exposed at the etched portion to cause oxidation in such exposed portion. Therefore, it has been difficult to reduce stress. Particularly, if the pattern is finer, the exposed area is larger to make influence of stress due to oxidation more significant. Also, once stress is caused, even if adjustment is attempted by annealing, ion implantation and so forth in the subsequent step, stress distribution associating with pattern size and/or pattern density has bear caused. Therefor, it is difficult to reduce stress over the entire area of the X-ray absorber.
In addition, upon formation of the resist pattern (resist mask) for forming the X-ray absorber pattern by way of electron beam drawing method, there has been proposed a technology for correcting the pattern drawing in consideration of strain of the pattern which may be produced during the process (No. 43.sup.th Applied Physics Relating Lecture Meeting Preliminary Report, 27p-zp-9). However, even with such technology, strain of the pattern cannot be solved completely.
Furthermore, there has been proposed to prevent the X-ray absorber from being exposed to the ambient air and, at least to prevent oxidation after patterning, by forming anti-oxidation layer on the X-ray absorber. However, such coat layer should influence for permeability and reflection of X-ray to lower contrast of the line exposure to reduce alignment accuracy.